High density integrated circuit (IC) packages that house LSI/VLSI type semiconductor devices are well known. Input/output contacts for such IC packages are often arranged in such a dense pattern (sometimes more than five hundred closely spaced contacts) that direct soldering of the IC package to a substrate, such as a printed wiring or circuit board (PCB) creates several significant problems related to inspection and correction of any resulting soldering faults as well as thermal expansion mismatch failures.
Land grid array (LGA) connectors are known for interconnecting IC packages to PCB's. LGA's typically do not require soldering procedures during engagement with the PCB. Referring to FIG. 1, prior art LGA assemblies are used to interconnect an IC package A having a plurality of contact pads or bumps B formed on a bottom surface, to contact pads C arranged in a regular pattern on a surface of printed wiring board or printed circuit board (PCB) D. Current technology permits conductive pads B and conductive pads C to be disposed at center-to-center spacings (as indicated by dimension “a” in FIG. 1) of approximately one half to one millimeter, with further miniaturization possible and inevitable.
Prior art LGA assemblies E are known which include an insulative housing and a plurality of resilient conductive contacts F received in passageways formed in the housing. The resilient conductive contacts typically have exposed portions at the upper and lower surfaces of the insulative housing for engaging flat contact pads B,C. When IC package A is accurately positioned in overlying aligned engagement with PCB D, such that conductive pads B engage conductive pads C, a normal force is applied to the exposed portions of each resilient conductive contact to electrically and mechanically engage the respective contact pads.
The resilient conductive contacts associated with prior art LGA's have had a variety of shapes. A commonly used form of resilient conductive contact includes two free ends connected by a curved portion which provides for the storage of elastic energy during engagement with the IC package and PCB. Prior art resilient conductive contacts are usually a single metal structure in the form of a spring to provide the required elastic response during service while also serving as a conductive element for electrical connection. Typically, a combination of barrier metal and noble metal platings is applied to the surface of the spring for corrosion prevention and for electrical contact enhancement. It is often the case that these platings are not of sufficient thickness for electrical conduction along the surface of the spring. Examples of such prior art resilient conductive contacts may be found in U.S. Pat. Nos. 2,153,177; 3,317,885; 3,513,434; 3,795,884; 4,029,375; 4,810,213; 4,820,376; 4,838,815; 4,922,376; 5,030,109; 5,061,191; 5,232,372; and 5,473,510. The foregoing patents are hereby incorporated herein by reference.
A problem exists in the high density electrical interconnection art in that a good material for the construction of a spring, such as a high strength steel, is not a very good electrical conductor. On the other hand, a good electrical conductor, such as a copper alloy or precious metal, is often not a good spring material. There is a need for a simplified resilient conductive contact which incorporates the seemingly opposing requirements of good spring properties and high conductivity. Additionally, attributes, missing from the prior art that are necessary for a universally applicable electrical contact include: (i) extendibility to a large contact array at fine pitch, i.e., five mils (thousandths of an inch) or less and (ii) spring members of relatively small size but high elastic compliance, i.e., spring members capable of deflections in the elastic range of as much as thirty percent of their uncompressed or undeflected height, and with low contact force, i.e., less than twenty grams per contact. In addition, such a universally applicable electrical contact will be capable of high frequency transmittance of signals greater than ten gigahertz, which would require a small self-inductance and therefore a short contact height. Also, a universally applicable electrical contact will be capable of high current capacity, i.e., having less than ten milliohm bulk resistance per contact and low contact resistance. Furthermore, a universally applicable electrical contact will be capable of high durability or high cycles of touchdowns, i.e., greater than five hundred thousand cycles, which requires a spring having a high elastic compliance to avoid permanent set in contact height under repeated compressive loadings as well as high fatigue strength. Additionally, a universally applicable electrical contact will be capable of high reliability with minimum degradation in contact resistance which often requires a noble metal contact surface and redundancy in contact points. Also, a universally applicable electrical contact will be capable of high service temperatures, i.e., often exceeding two hundred and fifty degrees centigrade, which requires the structural part of the electrical contact to be made of high melting temperature metals to prevent the relaxation of contact force. All of the foregoing will be essential, but will only help solve the problems in the art if achieved with low cost manufacturing, using conventional high volume tools and processes.
Therefore, an improved electrical contact system and assembly for use in a wide variety of electrical connector and interface sockets and interposers is needed which can overcome the drawbacks of conventional electrical contacts and exhibit the foregoing attributes.